In 2007, the Renewable Fuels Association conducted a survey of U.S. ethanol plants. The survey collected a variety of information, including data on water usage. Twenty-two ethanol plants representing more than 1.8 billion gallons of ethanol production responded to the survey. These plants accounted for 37 percent of total U.S. ethanol production in 2006.

The Argonne National Laboratory analyzed the RFA's data and compared it with a 2003 USDA survey of ethanol plants, and according to the report released in March "Analysis of the Efficiency of the U.S. Ethanol Industry 2007," water consumption at dry-grind ethanol plants decreased 26.6 percent. The report says dry-grind ethanol plants currently use an average of 3.45 gallons of fresh water per gallon of ethanol produced, while wet-mill ethanol plants use an average of 3.92 gallons of water per gallon of ethanol. According to USDA's 2003 study, ethanol plants used an average of 4.7 gallons of water to make 1 gallon of ethanol. The study didn't separate wet- and dry-grind plants.

The ANL analysis also found there are significant variations in water use among individual plants. Dry-grind ethanol plants reported water usage statistics ranging from 2.65 gallons of water per gallon of ethanol to 4.9 gallons per gallon of ethanol. Wet-mill plants reported water use varied from a minimum of 1.2 gallons of water per gallon of ethanol to a maximum of 6.1 gallons per gallon of ethanol.

Current State of Water Use
Large variations in water usage can be explained by a variety of factors. According to the ANL report, newly constructed plants tend to require less water, primarily due to improved designs. According to Greg Hausmann, ICM Inc.'s vice president of engineering, the quality of incoming water, process considerations, utility system configuration and equipment selection considerations also affect a plant's water needs. "We believe that the industry is doing very well, and making a big effort to reduce water use, either through recycling or process change," says May Wu, an ANL environmental scientist who authored the March report. "There are a lot of things ongoing, and newly built plants are definitely much more water efficient."

According to Andy Aden, a National Renewable Energy Laboratory senior research engineer, energy and water demands at ethanol plants are closely integrated. "Anything you can do to make your plant more energy efficient is going to help your water usage as well," he says.

Aden wrote a journal article in 2007 titled "Water Usage for Current and Future Ethanol Production" about water use in ethanol plants. "The real take-home message of the paper was that the process water—meaning the water that is in fermentation that goes through the normal part of the process—is really not the issue," he says. "The demand … is in the utilities side. It is water lost to evaporation from the cooling tower. It is water lost to blowdown in the boiler system. It is all these extraneous water uses that have to be made up, and that's where the improvements lie."

Carolyn Kotsol, director of process technology for Delta-T Corp., says the first step in reducing water usage is to identify the plant's largest water consumers. "First and foremost, I think most technology providers have really refined the amount of indirect heat that the plant uses," she says. "Increasing the amount of indirect heat that you use in the design decreases the amount of fresh water you need for your boiler. That helps significantly."

Another primary place where water is lost is through dryer exhaust, Kotsol says. If you can capture that exhaust, you are harnessing heat. You can take the vapor, condense it into a liquid stream and then reuse it in the plant to offset some of the biggest water consumers.

"The largest user of fresh water in the plant is the cooling tower," Kotsol says. One option to reduce that water use is to offset the fresh water used in the cooling tower with another stream, such as dryer exhaust. "Then you could greatly reduce the amount of fresh water that you are bringing into the plant," she says. "That technology is out there, and it has been applied in a few plants." This technology can offset total water usage at the plant by approximately 1 gallon of water per gallon of ethanol.

Kotsol says as the industry recovers from the high corn prices of 2008, many water-saving technologies are being implemented at existing facilities rather than through new construction. Optimization is often a great option for existing plants, she says. "They may have already done some troubleshooting and some de-bottlenecking, so this is the time to go out there and optimize the plant from a water usage standpoint as well," Kotsol says.

According to Erin Heupel, Poet LLC's design and construction lead environmental engineer, her company is trying to incorporate water savings technologies into all future plant designs, and retrofitting existing plants. "We're really re-evaluating all the water streams in a plant and looking for areas to recycle or reduce to be more efficient in our water usage," she says.

Heupel recently studied Poet's plants and found that on average they are using less than 3 gallons of water to produce a gallon of ethanol. "Depending on the source water quality, you'll see variability," she says. The average was calculated by comparing Poet's total ethanol production against the total volume of water used at the plants. In five years, she hopes the facilities will be closer to using 2 gallons of water per gallon of ethanol. "This is dependent on the initial water quality," Heupel says. "I think the trend we hope to see in our plants is that we can eventually take them to zero-liquid."

ICM has also been working on technologies to minimize water usage in its plant designs. "[ICM] has designed a couple of zero-discharge facilities that are in various stages of construction," Hausmann says. However, the technologies that reduce water usage often carry high capital costs. "When we bring [capital cost] considerations into play, then our technology options start getting limited," Hausmann says.

Keith Scarberry, ICM's water systems engineering manager, and Hausmann agree that one key element to reducing water usage is water quality. "The better the water quality is, the less water will be consumed by the factory," Hausmann says.

Water Use in the Future
Interest in reducing water use at ethanol plants is expected to continue. "During the time I've been with ICM [water quality and usage] has gone from almost an afterthought by the owners and project developers to a consideration most of them are taking with their site selections," Scarberry says. Wu says that through her attendance at numerous ethanol conferences, she has seen that many existing plants are working to further minimize water usage. "I have heard more and more talk about water recycling," she says. "The key limitation, perhaps, is the cost." Many technologies, however, are getting more affordable. "I am pretty confident zero-water discharge can be realized," Wu says.

Plant designers and technology providers have developed several methods to reduce water usage, including the replacement of evaporative cooling towers with air exchangers or other cooling media. "The technology folks have taken some pretty significant steps in the right direction to reduce the amount of water consumption in our plants," Kotsol says. Those new technologies won't necessarily eliminate the use of cooling towers, but they would certainly reduce the size of the cooling tower need, which would reduce the amount of fresh water required by the plant. "It's certainly a known technology used in other industries," she says.

While most involved with the industry agree water use at traditional corn-based ethanol plants will continue to decline, it is not clear how much water cellulosic technologies will require. "In our studies we have seen numbers as low as 1 gallon of water per gallon of ethanol, and up to 9 to 10 gallons per gallon," Wu says. "That's a wide range. It depends on the process you select."

Although no commercial-scale cellulosic ethanol plants currently exist, NREL research suggests the range of water use by cellulosic facilities will be similar to what is seen with corn ethanol. "The cellulosic numbers are based on our very best detailed modeling efforts," Aden says.

According to Aden, a 2002 NREL report came up with a figure of 6 gallons of water per gallon of ethanol at a cellulosic plant. However, a model optimized for water efficiency yielded as little as 1.9 gallons of water per gallon of ethanol in 2007.

Kotsol says that analyzing the amount of water coming into a cellulosic ethanol plant will be a primary consideration during the design process. "It will constantly be a part of our
technology challenge," she says.

Unlike the early days of the corn-based ethanol plant development, water usage will be a chief concern when developing cellulosic ethanol plants. "There are still a lot of unknowns with [cellulosic ethanol]," Hausmann says, which makes it hard to predict how water will be utilized. "I think it's safe to say that nobody doing a cellulosic system investigation effort, or development effort, is doing it without consideration of water consumption."

Erin Voegele is an Ethanol Producer Magazine staff writer. Reach her at evoegele@bbiinternational.com or (701) 373-8040.